Alexei Navalny, Russian President Vladimir Putin's most formidable domestic opponent, fell unconscious and died after a walk at the "Polar Wolf" Arctic penal colony where he was serving a three-decade sentence, authorities said. The death of Navalny, a 47-year-old former lawyer, robs the disparate Russian opposition of its most courageous leader as Putin prepares for an election which will keep the former KGB spy in power until at least 2030. There are now no opposition leaders of such prominence left in Russia. For some young urban Russians, Navalny offered hope of an alternative future to Putin, who has served as Russia's…